Lamp, filamentary wire and method of making said wire



Jan. 22, 1963 R. H. SCHNITZEL 3,075,120

FILAMENTARY WIRE AND METHOD OF MAKING SAID WIRE Filed Dec. 24, 1959 2 Sheets-Sheet 1 FIG. 7.

.40 EM -L I l-J I EE mm SAG IN 2.6 cm

l0 '20 3O 4O 5O 60 TIME IN MINUTES I N V EN TOR. FHA/00L PH H. $(///v/ 775A WQW Jan. 22, 1963 R. H. SCHNITZEL 3,075,120

LAMP, FILAMENTARY WIRE AND METHOD OF MAKING SAID WIRE Filed D60. 24, 1959 2 Sheets-Sheet 2 INVENTUIZ R/M/DOLF'H H. SCHN/TZEL -WOW operation.

overlap one another.

hired fi rates This invention relates to incandescent lamps, tungsten wire and method and, more particularly, to recrystallized tungsten filamentary Wire having a particular microstructure, a lamp incorporating such wire and a method for making the wire.

Tungsten has been used as a filamentary wire for incandescent lamps for many years. In accordance with the teachings contained in US. Patent No. 1,082,933, dated December 30, 1913, the wire normally contains limited amounts of so-called additive doping impurities, order to prevent filament creep or sag during lamp While such so-called non-sag filament wire is generally satisfactory, it would be desirable to improve still more the non-sag characteristics of tungsten filament wire.

it is the general object of this invention to provide a tungsten filamentary wire which in recrystallized form has improved non-sag characteristics.

it is a further object to provide an incandescent lamp incorporating recrystallized tungsten filamentary Wire which has improved non-sag characteristics.

It .is an additional object to provide a method for processing tungsten filamentary wire in order to improve the non-sag characteristics of the wire.

The aforesaid objects of the invention, and other objects which will become apparent as the description proceeds, are achieved by providing a recrystallized tung sten filamentary wire having a microstructure which comprises a plurality of generally parallel crystals which are elongated in the general direction of the wire axis and The boundaries which define these crystals have a generally-spiraled orientation with respect to the wire axis and these spiraled and interlocking crystals improve the non-sag characteristics of the wire. There is also provided a method for processing such tungsten filamentary wire wherein the wire, prior to recrystallization, is torsionally stressed a sufiicient amount to eficct therein a permanent and substantially uniform strain.

For a better understanding of the invention, reference should be had to the accompanying drawings wherein:

FIG. 1 is an elevational view, partly in section, of an incandescent lamp incorporating the improved filamentary wire of this invention;

PEG. 2 is a photomicrograph taken of tungsten filamentary wire atter twisting in accordance with the present invention, but before recrystallization;

FIG. 3 is a photomicrograph taken of tungsten filamentary wire after twist ng and limited recrystallization;

FIG. 4 is a photomicrograph taken of tungsten filamentary wire similar to FIG. 3, but after more extensive recrystallization;

FIG. 5 corresponds to FIG. 4 except that the photomicrograph was taken at a much lower magnification;

HG. 6 is a photomicrograph taken for non-sag tung- .sten filamentary wire prepared and then recrystallized in accordance with the practices of the prior-art;

KG. 7 is a graph of sag versus time, illustrating the improved non-sag characteristics which are obtained with the present wire.

While the wire of the present invention can be used in any type of device wherein a tungsten wire is heated to incandescence, the present wire has particular utility atent 'vention, the tungsten filamentary wire is first prepared in dfi'iilzfi Patented Jan. 22, 19%3 ice with respect to large size incandescent lamps and hence it has been so illustrated and will be so described.

With specific reference to the form of the invention illustrated in the drawings, in FIG. 1 is shown a 750 watt incandescent lamp 10 which generally comprises a glass envelope 12 having a base 14 secured thereto. The neck of the envelope has sealed thercthrough a reentrant stem press 16 and lead conductors 18 are sealed through the stem press 16 and support a vertically-disposed filament as between their inwardly-extending extremities. Since this type of filament is particularly subject to sag during lamp operation, the filament 28 is separated into two series-connected sections supported by a bridge 22, which in turn is supported by the lead wires 18. The filament could be formed in one section if desired. For this type of lamp, it is desirable to use a heat shield 24, supported by the stem press 16, to prevent excessive heating of the base portion of the lamp when it is burned in a baseupward position. The filament Zil desirably has a coiledcoil configuration and has been processed in accordance with the present invention as explained hereinafter. As an example, the diameter of the filament wire is 9.3 mils, the wire in each coil half has a length of 281 mm. and the each coil section comprising the filament has a length of 31 mm. The lamp contains an inert gas fill of argon and 19% neon at a pressure of 600 mm. mercury. While a specific lamp has been described in detail, the present filamentary wire can also be used in other designs of lamps.

Before processing in accordance with the present inaccordance with conventional practices. As is known, tungsten filamentary wire for use in incandescent lamps is prepared in accordance with powder metallurgy practices wherein tungstic acid or tungstic oxide, as prepared from tungsten ore, has added thereto predetermined amounts of specific additive doping impurities or compounds. The purpose of these additives is to inhibit the .so-called sag or creep tendencies of the tungsten filament. As an example, silicon dioxide, alumina and potassium chloride are added to tungstic oxide or tungstic acid as potassium silicate and aluminum chloride with an addition of a small amount of hydrochloric acid to precipitate silica in a very uniform fashion throughout the tungsten compound. The percentages of such added doping compounds can vary considerably and other known doping compounds can be substituted for those indicated. As an example, the percent by weight of silicon in the doping compounds, as expressed in terms of equivalent silicon dioxide, is preferably about 4% by weight of tungsten, .as expressed in terms of tungstic acid. The

percent by weight of aluminum-in the doping compounds, as expressed in terms of equivalent aluminum trioxide,

is preferably about 0.03% by weight of tungsten, as

preferably about 0.5% by weight of tungsten, as expressed in termsot tungstic acid. After the doping compounds .are added, the tungstic oxide or acid is reduced to metallic tungsten powder. The resulting doped powder is compressed to form aso-callcd compact or green ingot and the ingot is electrically sintered until it fuses into a cohesive mass, After sintering, the ingot is swaged to wire of predetermined diameter and the resulting swaged wire is then drawn through a series of-dies to reduce the wire diameter to a predetermined value suitable for a lamp filament. Annealing steps are utilized at various points in the swaging and drawing process. The foregoing conventional tungsten proccssing is generally described in aforementioned "US. Patent No. 1,082,933, dated December 30, 1913, and a specific suitable swagto a predetermined diameter, has placed thereon a torsional stress which is sufiicicnt to effect therein a substantially uniform and permanent torsional strain. Otherwise expressed, the drawn wire of predetermined diameter is twisted along its axis an amount sufiicient to place therein a permanent strain, but insufficient to cause any discrete reduction in wire diameter, which reduction would manifest itself as a necked portion. As a specific example, tungsten wire having a diameter of 9.3 mils is twisted from 180 to 270 per millimeter of wire length. By way of further example, a 600 mm. length of such wire is twisted for a total of 390 to 450 turns at a rate of 2 to 60 rpm. This will cause the wire to have 'a permanent and substantially uniform torsional strain. As is well known in the art, the so-called microstructure of tungsten wire is readily determined by first polishing a longitudinal cross section of the wire, etching the wire cross section with acid in order to define the crystal or incipient crystal boundaries and observing the etched wire through a microscope. The microstructure of wire twisted in accordance with the foregoing example is shown in FIG. 2 which is a 200x photomicrograph of such twisted wire prior to recrystallization. The axis of the wire as shown in FIG. 2 is from the bottom to the top of the figure and the wire microstructure shows that the etched incipient crystal boundaries, which represent a so-called fiberous or worked structure, are generally spiraled with respect to the wire axis. In the usual unrecrystallized non-sag wire,'the, socalled worked structure is similar to that shown in FIG. 2 except that the incipient crystal boundaries which comprise the worked structure actually parallel the wire axis and are not spiraled with respect to the wire axis.

The present twisted wire can be recrystallized before it is incorporated into a lamp. In FIG. 3 is shown a 200x photomicrograph of the etched twisted wire after it is heat treated in vacuum by passing therethrough a current of 3 amps. for thirty minutes and thereafter increasing the heating current to 5 amps, and maintaining this current for a period of five minutes. This heat treatment causes the wire to' recrystallize and the crystals which comprise the microstructure of the wire are elondetermined conditions in order to cause the filament to recrystallize. The so-called flashing schedules are well known in the art and the usual flashing schedules which are used with untwisted but otherwise similar wire are suitable for recrystallizing the present twisted wire; By way of further example, the filament in the lamp as shown in FIG. 1 and as described hereinbefore is recrystallized by applying'thereacross for a period of 4 to 6 seconds a limited voltage corresponding to 9% of the 115 volt designed lamp operating voltage. Thereafter the lamp operating voltage is increased to its normal 100% value and this voltage is maintained for a period of 4 to 6 sec ends. The voltage applied across the filament is then increased to 120% of the designed operating value and this excessive voltage is maintained for a period of 6 to 9 seconds. This schedule causes the filament t0 recrystallize properly and the resulting recrystallized structure corresponds to that shownin FIGS. 4 and 5. As indicated, the foregoing filament flashing or recrystallization schedules are subject to considerable variation depending upon the type of lamp, the designed lamp wattage and intended lamp operating voltage and such schedules are well known.

In FIG. 6 is shown the conventional recrystallized microstructure for the usual non-sag tungsten filament which is not twisted in accordance with the present invention. In this microstructure which represents a mag nification of 209x, the crystal boundaries are generally parallel and elongated in the general direction of the wire axis and also overlap one another to interlock. As shown, however, these interlocking crystals are not spiraled with respect to the wire axis.

The curves designated as A and B in FIG. 7 illustrate comparative sag or creep data for a 9.3 diameter tungsten wire previously twisted and then recrystallized as shown in FIGS. 4 and 5 and a 9.3 mil diameter nonsag tungsten wire recrystallized as shown in FIG. 6. Except for the twisting as specified hereinbefore, these tungsten wires were identical. In taking the data to establish these curves, a 2.6 centimeter length of each of the compared wires was suspended horizontally between two sup ports in vacuum (pressure less than 1 micron). Each wire was incandesced by passing therethrough a current of 5 amps. and the millimeters sag (deviation from the horizontal) at the center of each of the wires recorded.

' After five minutes operation under these conditions, both gated in the general direction of the wire axis and overi lapping one another, with the boundaries which define the crystals having a generally spiraled orientation with respect to the wire axis. A comparison of FIGS. 2 and 3 will show that the degree of spiral of the incipient crystal boundaries and the degree of spiral of the formed crystal boundaries are generally the same. In order to complete the recrystallization to the desired degree, the wire as shown in FIG. 3 is additionally heated in vacuum by passing therethrough a current of 7 amps. for a period of 3 minutes. The resulting microstructure of the recrystallized wire is shown in FIGS. 4 and 5, wherein the magnification was 200 and X respectively. The actual surface temperatures of the wire during the foregoing specific recrystallization schedule are about 1550 C. for a current of 3 amps, about 2120 C. for a current of 5 amps. and about 2750 C. for a current of 7 amps.

In the usual case, the drawn and twisted tungsten wire will be formed, before recrystallization, into a coil or a coiled-coil filament and incorporated into an incandescent lamp, although some limited lamp types use an uncoiled filament. After lamp fabrication is completed, the filament is recrystallized by heating it to a predetermined temperature for a predetermined time and this is known in the art as flashing the lamp. As an example, the 9.3 mil diameter wire, previously twisted as described, is formed into a coiled-coil and incorporated into the lamp shown in'FlG. 1. After lamp fabrication is completed, the filament is flashed by energizing the lamp under prewires under test displayed a maximum sag. As shown in FIG. 7, however, the maximum sag realized with the present twisted non-sag wire was only seventy-five percent that realized with the normal untwisted non-sag wire. Even better comparative improvements have been realized in large wattage incandescent lamps which use a verticallydisposed filament, where the problems of filament sag or creep are accentuated.

The foregoing wire processing is subject to considerable variation. For example, it is very desirable, although not mandatory, to stress relieve the drawn wire before it is twisted in accordance with the present invention. Stress relief is accomplished by heating the wire at a predetermined temperature below its recrystallization temperature for a predetermined time. The timetemperature relationships' required to relieve stresses will vary depending on the typeof wire processed, its diameter and previous drawing or working history. The required time-temperature relationships to stress relieve such wire are well known in the art. As a specific example, the specific 9.3 mil wire considered herein can be stressed relieved by heating same in a non-oxidizing atmosphere such as hydrogen at a temperature of 1500 C. for a period of approximately five minutes. Where the wire has been stress relieved before it is twisted in accordance with the present invention, there will be less tendency for necking. Also, while the present specific wire is permissibly twisted at room temperature, heating the wire during twisting will decrease the tendency to develop any uneven strain pattern which can produce necking at a discrete location. similarly, cooling the wire during twisting will accentuate tendencies for necking. The foregoing twist of from 180 to 270 per millimeter of wire length is quite satisfactory and will produce a desired substantially uniform permanent strain in the wire. The actual degree of twist is not critical, however, and good results have been obtained where the wire was twisted through angles of from 90 to 180 per millimeter of wire length.

Various additive materials have been found to improve the susceptibility of the wire to twist readily. As a specific example, a molybdenum additive of 0.25% by weight of the tungsten causes the Wire to take a permanent and substantially uniform torsional strain more readily. As a further alternative to the present method, the Wire can be further drawn to a limited degree after the worked structure has been spiraled with respect to the wire axis, as shown in FIG. 2.

In copending application S.N. 819,392, filed June 10, 1959, and owned by the present assignee, is disclosed an undoped, substantially pure tungsten which recrystallizes with an interlocking non-sag structure similar to that disclosed in FIG. 6 of the present application. While the wire of the present invention will normally have doping impurities or compounds deliberately added thereto, it should be understood that the present wire can be processed from so-called undoped tungsten which has been purified in accordance with the teachings of the aforementioned copending application S.N. 819,302, filed June 10, 1959. The undoped and substantially pure tungsten wire after twisting in accordance with the present invention and later recrystallization will display a miscrostructure comprising crystals having boundaries which are generally spiraled with respect to the wire axis, as shown in FIGS. 3 through 5.

It will be recognized that the objects of the invention have been achieved by providing a tungsten filamentary Wire having improved non-sag characteristics and there has also been provided an incandescent lamp incorporatin recrystallized tungsten filamentary Wire which has improved non-sag characteristics. There has also been provided a method for processing tungsten filamentary wire in order to improve its non-sag characteristics.

While best embodiments of the invention has been illustrated and described hereinbefore, it is to be particularly understood that the invention is not limited thereto or thereby.

I claim:

1. A recrystallized and generally uniform tungsten filamentary wire, the microstructure of said wire comprising a plurality of generally parallel crystals elongated in the general direction of the wire axis and overlapping one another, substantially all of the boundaries defining the crystals comprising the microstructure of said wire having a generally uniform spiraled orientation with respect to the wire axis, and said wire having a smooth and continuous surface.

2. In combination with an incandescent lamp, a recrystallized and generally uniform tungsten filamentary wire of substantially uniform diameter, the microstructure of said wire comprising a plurality of generally parallel crystals elongated in the general direction of the wire axis and overlapping one another, substantially all of t e boundaries of the crystals comprising the microstructure of said wire having a generally uniform spiraled orientation with respect to the wire axis, and said wire having a smooth and continuous surface.

3. A recrystallized tungsten filamentary wire of substantially uniform diameter and suitable for use in incandescent lamps, the axis of said wire having a coiled configuration, the microstructure of said wire comprising a plurality of crystals elongated in the general direction of said wire axis and overlapping one another, substantially all of the boundaries of the crystals comprising the microstructure of said wire having a gen- 6 erally uniform spiraled orientation with respect to said wire axis, and said wire having a smooth and continuous surface.

4. A recrystallized tungsten filamentary wire of substantially uniform diameter and suitable for use in incandescent lamps, the axis of said Wire having a coiledcoil configuration, the microstructure of said wire comprising a plurality of crystals elongated in the general direction of said wire axis and overlapping one another, substantially all of the boundaries of the crystals comprising the microstructure of said wire having a generally uniform spiraled orientation with respect to said wire axis, and said wire having a smooth and continuous surface.

5. The method of processing, prior to recrystallization, tungsten filamentary wire prepared initially by mechanically reducing a sintered tungsten ingot to wire having a predetermined diameter substantially corresponding to the final desired diameter for the wire, which method comprises, placing on said wire a torsional stress sufiicient to effect therein a permanent strain but insufficient to efliect therein any discrete reduction in wire diameter.

6. The method of processing, prior to recrystallization, substantially pure tungsten filamentary wire prepared initially by mechanically reducing a sintered tungsten ingot to Wire having a predetermined diameter substantially corresponding to the final desired diameter for the Wire, which method comprises, placing on said wire a torsional stress sufiicient to effect therein a permanent strain but insufiicient to effect therein any discrete re duction in wire diameter.

7. The method of processing, prior to recrystallization, tungsten filamentary wire prepared initially by me chanically reducing a sintered tungsten ingot to wire having a predetermined diameter substantially corresponding to the final desired diameter for the wire, which method comprises, heating said wire to relieve substantially all stresses therein, and placing on said wire a torsional stress sufficient to effect therein a substantially uniform and permanent strain.

8. The method of processing, prior to recrystallization, tungsten filamentary Wire prepared initially by mechanically reducing a sintered tungsten ingot to wire having a predetermined diameter substantially correspending to the final desired diameter for the wire, with the tungsten compact before sintering into ingot form containing predetermined amounts of predetermined impurities as required to form elongated and interlocking tungsten grains within the formed wire when it is later heated to a predetermined recrystallization temperature, which method comprises, placing on said wire a torsional stress suflicient to effect therein a substantially uniform and permanent strain.

9. The method of processing, prior to recrystallization, tungsten filamentary wire prepared initially by mechanically reducing to wire having a predetermined diameter substantially corresponding to the final desired diameter for the Wire a sintered tungsten ingot, with the tungsten compact before sintering into ingot form containing predetermined amounts of aluminum-, potassiumand silicon-containing impurities as required to form elongated and interlocking tungsten grains within the formed wire when it is later heated to a predetermined recrystallization temperature, which method comprises, placing on said wire a torsional stress sufiicient to effect therein a substantially uniform and permanent strain.

10. The method of processing tungsten filamentary Wire prepared initially by mechanically reducing to wire having a predetermined diameter substantially corresponding to the final desired diameter for the wire a sintered tungsten ingot, with the tungsten compact before sintering into ingot form containing predetermined amounts of predetermined impurities as required to form elongated and interlocking tungsten grains within the formed wire when it is later heated to a predetermined recrystallization temperature, which method comprises,

heating said wire to relieve substantially all stress therein, placing on said wire a torsional stress sufficient to efiect therein a substantially uniform and permanent strain, and thereafter heating said Wire at a predetermined temperature and for a predetermined time to 'cause said Wire to recrystallize.

11. The method of processing, prior to recrystallization, tungsten filamentary wire prepared initially by mechanically reducing to wire of 0.0993 inch diameter, a sintered tungsten ingot, with the tungsten compact before sintering into ingot form containing predetermined amounts of aluminum-, potassiumand silicon-contain- References Cited in the file of this patent V UNITED STATES PATENTS 1,461,117 Hall July 10, 1923 1,670,292 Blaw et al. May 22, 1928 1,720,006 Ramage July 9, 1929 2,250,610 Simone July 29, 1941 

10. THE METHOD OF PROCESSING TUNGSTEN FILAMENTARY WIRE PREPARED INITIALLY BY MECHANICALLY REDUCING THE WIRE HAVING A PREDETERMINED DIAMETER SUBSTANTIALLY CORRESPONDING TO THE FINAL DESIRED DIAMETER FOR THE WIRE A 